1. Field of the Invention
A widely practiced, commercially important method for manufacturing phosphoric acid from phosphate rock, referred to hereinafter for the sake of convenience as the conventional wet process, and more simply as the wet process, comprises the reaction of phosphate rock with sulfuric acid to produce phosphoric acid and by-product calcium sulfate. In such conventional wet process, the phosphate rock feed material is finely ground and subsequently digested in a mixture of sulfuric and recycled phosphoric acids for a relatively long time (e.g., four to six hours) and at an elevated temperature (e.g., 75.degree. C., and above) in order to achieve high conversion of the mineral phosphate values therein to phosphoric acid and to produce, as a by-product therefrom, calcium sulfate crystals which can be filtered and washed efficiently from mother liquor. Satisfactory and economical operation of such conventional wet processes requires a relatively high purity phosphate rock feed material; low purity rock feeds cause many known difficulties in the rock grinding, acidulation, filtration, filter cake washing, and other operations of the process, and high levels of impurities dissolved in the acids produced from low purity rock cause difficulties in the subsequent manufacture of phosphate fertilizers from such wet-process phosphoric acid.
In order to prepare phosphate rock which meets conventional wet-process feed material purity specifications, most naturally occurring phosphate ore must first be beneficiated by a combination of washing, sizing, flotation, calcination, and like treatments to separate the phosphatic constituent from impurity minerals and organic contaminants. A significant consideration attendant to such ore preparation is the fact that substantial amounts of the phosphate values contained in the ore normally are not recovered by these treatments, i.e., they are lost from the process feed stream. As a consequence of such consideration, prior-art researchers have devoted considerable attention to minimizing the rather large amounts of phosphate values that are discarded with the minus 150-mesh slime material generated during such beneficiation of, for instance, Florida phosphate ore. In addition, such prior-art researchers have long considered and attempted to solve the problems associated with losing appreciable amounts of phosphate which occurs during further beneficiation treatments of deslimed materials, particularly during phosphate rock flotation operations. Typically, about 20 percent of the phosphate contained in deslimed flotation feeds (materials sized between about minus 14-mesh and plus 150 mesh) is discarded with the flotation mill tailings. Flotation of coarse material (about minus 14-mesh to about plus 35 mesh) is particularly inefficient; about 50 percent of the phosphate content in the coarse flotation feed is not recovered.
Not all phosphate rock products from the beneficiation treatments are sufficiently pure to be used as conventional wet-process feed materials; as for example, low-grade phosphate rock pebble in which silicon-, iron-, and/or aluminum-containing mineral inclusions render the product grade and its ratio of impurities-to-phosphate unacceptable for use as acid plant feed. Although these materials might be crushed to liberate phosphate from the included impurities and then subjected to further beneficiation to recover the librated phosphate, this will increase the amounts of phosphate that are lost in the form of minus 150-mesh materials and material lost by inefficiencies in the flotation operations. It is also possible, and a common practice in the industry, to blend lower and higher purity phosphate rock so that the average composition of the blend meets specified purity levels. However, as will be described in the following section, rock blending does not provide a universal means to expand the capabilities of the conventional wet process to allow recovery of phosphate values from any low purity rock product.
2. Description of the Prior Art
Numerous prior-art references describe alternatives to the conventional wet process which are applicable to the production of phosphoric acid from low purity phosphate rock. One generally disclosed method is a sequential type of wet process which comprises reacting phosphate rock with recycled phosphoric acid to form a solution phase of monocalcium phosphate dissolved in phosphoric acid and a solid phase of undissolved gangue material, separating the undissolved gangue from the solution, reacting the monocalcium phosphate solution with sulfuric acid to produce a solid calcium sulfate and phosphoric acid solution, separating the phosphoric acid solution from the calcium sulfate, recycling a portion of the phosphoric acid solution to the rock acidulation stage, and withdrawing the remaining phosphoric acid solution as product. As examples of sequential wet processes, see U.S. Pat. Nos. 2,531,977, Hammaren et al., Nov. 28, 1950; 3,619,136, Case, Nov. 9, 1971; 4,435,370, Holcomb et al., Mar. 6, 1984; 4,029,743, Hauge, June 14, 1977, and the references cited therein. The rock acidulation conditions of these processes are mild relative to those of the conventional wet process. Control of temperature, ratio of acid to rock, reaction time, concentration of dissolved fluorine, and like conditions to achieve high solubilization of phosphate contained in the rock feed with minimal coextraction of impurities is taught in the disclosures of sequential wet processes mentioned above as well as in U.S. Pat. No. 3,843,767, Faust et al., Oct. 22, 1974.
It will be appreciated by those skilled in this art that prior art wet processes of the sequential type do not provide the means for extending the capabilities of existing conventional wet-process phosphoric acid production facilities to accept phosphate values from lower purity phosphate rock feeds. Rather, the processes of the prior art are taught to be operated as separate facilities in place of existing, convenional wet-process acid production facilities. Practice of these prior-art teachings would involve separate operations of sequential and conventional acidulation facilities to process the lower and higher purity feeds, respectively, that are produced at a given mining and beneficiation facility, and these separate operations would require costly replications of equipments, particularly gypsum filtrations and wash equipment, acid pumps, and facilities to store the recycle and the product acids. Fortunately, these costly replications are avoided in the process of the instant invention.
It also will be appreciated that the stated intent of many prior art sequential-type processes (e.g., '370 and '136 supra) is a much or more directed to the recovery of by-product fluorine-containing materials as to the efficient production of phosphoric acid from the available resources of higher and lower purity phosphate rock. Defluorination treatments that are a vital part of inventions such as that of '370 supra render the resultant solutions of calcium phosphate in phosphoric acid unsatisfactory for incorporation into the feed streams of an existing, conventional wet-process phosphoric acid facility. It is well known that alkali fluosilicate salts are not totally insoluble in phosphoric acid [Sven-Eric Dahlgren in A. V. Slack (ed.), Phosphoric Acid, Vol. 1, Part 1, Marcel Dekker, Inc., New York, p. 113 (1968)]. Therefore, solutions generated after defluorination treatments such as that taught by Holcomb et al, supra will contain residual concentrations of alkali metal ions and alkali metal fluosilicate salts that are equal to the saturation concentrations of these salts. The presence of the alkali metal ions and fluosilicate salts will increase levels of scale formation on equipment used for cooling and evaporating operations that are essential in the wet-process method and the production of concentrated phosphoric acid. The increased buildup of fluosilicate scale on acid plant equipment will require frequent and inefficient shut downs of acid production operations for cleaning.
The prior art also teaches methods for incorporating phosphate values from low-grade, low purity phosphate rock into the feed streams of existing wet-process acid production plants by the practice of blending small amounts of the lower purity rock with higher purity feed rock. Difficulties in preparing reliably homogeneous mixtures of solid mixtures by blending cause discontinuities in the compositions of the blended feed rock streams, and these discontinuities create disturbances in the chemistry of the wet process which interfere with the process operator's ability to maintain a smooth, continuous production of phosphoric acid. It should be noted that blending does not reduce the total quantity of impurities that are associated with the low purity rock constituent. Rather, they are incorporated into the blend and report with it to the wet-process feed grinding and acidulation operations, which thereby restricts the amounts of lower purity rock that can be blended with the available supply of higher purity rock. In contrast, the method of the present invention provides a way to selectively reject a portion of the congeneric impurity of the low purity rock and, thereby, overcome problems of increased ratios of impurities to phosphate in feeds that are prepared by blending high and low purity rock. Furthermore, the instant method provides for the blending of phosphate values in the form of solutions rather than solids, and thereby overcomes problems associated with inhomogeneous feedstocks prepared by dry rock blending.